This invention relates to variable ratio frictional drive gears of the kind comprising basically two axially spaced torus discs or rotors, one serving as an input and the other an output, between which there is a set of circumferentially spaced drive rollers in frictional rolling contact with part toroidal surfaces on the discs, each roller being rotatably mounted in a bearing structure which can tilt about an axis at right angles to the axis of rotation of each roller so as to vary the distances from the gear axis at which the roller engages the two discs respectively, thus varying the drive ratio of the gear. The angle of tilt of the roller bearing structure as it controls the drive ratio of the gear, is called the ratio angle.
One way of changing the ratio angle is to provide means to apply a force to each of the roller bearing structures to move it generally tangentially with respect to the gear axis, and by allowing the rollers then to steer themselves towards a different ratio angle. The rollers are each mounted in their bearing structures in such a way that they are inclined at an angle to a plane perpendicular to the gear axis. This angle is called the caster angle. Gears of this general construction are referred to as gears with tangentially controlled roller bearing structures. Such a drive gear will for convenience herein be described as being of the kind specified.
This invention is particularly, though not exclusively, concerned with gears in which the plane of each roller, normal to the axis of rotation of the roller and passing through the points of contact of the roller with the two opposed torus discs, contains the axis about which the roller tilts, being tangential to the torus centre circle (i.e. the locus of the centre of the circle revolved to generate the torus) as distinct from gears in which the same plane for each roller is closer to the main axis of rotation of the gear.
The input must rotate in the direction in which it tends to drag each roller against the control force which controls the tangential position of the rollers. The caster angle must be such that each roller tilt axis is inclined away from the input disc in the direction of movement of the disc. This criterion arises out of the fact that stable operation at any given ratio angle occurs when the axis of rotation of each roller passes through the gear axis. Unless the caster angle is as just described, tangential displacement of a roller (by virtue of an increase or decrease in the load on the gear or in controlling fluid pressure) will result in the torus discs producing a steering force on the roller which will tilt the roller in the direction opposite to that which is required to move the roller axis back to intersect the gear axis, so that the roller will be moved away from, instead of towards, its new stable position.
In general, the larger the caster angle, the more stably the rollers tend to maintain their ratio angles and consequently the more reliably the apparatus operates. This is of particular importance when the apparatus is run at very high rotational speeds, perhaps up to 20,000 revolutions per minute, though there are operating conditions in which maximising the caster angle is not so important.
There have, in the past, been many attempts to achieve ease of adjustment of the rollers with reliable operation of the apparatus, that is with minimum wear and maximum power transmission from the input to the output, and while many of them are satisfactory, most have some short comings, being, particularly, not well suited for all operating conditions, though good in some.
It is the object of this invention to provide a transmission system of the kind specified in which the efficiency is maximised for a wide range of operating conditions.
According to the invention there is provided a transmission system of the kind specified wherein the means for moving each of the roller bearing structures generally tangentially of the gear axis is arranged to apply a force to said structure, said force being in a direction non-parallel with respect to the plane which is perpendicular to the gear axis, there being means for accommodating effective movement of the roller bearing axes relatively to the gear axis, in a direction parallel to the gear axis.
The invention will now be described by way of example with reference to the accompanying drawings in which:-